In order to reduce imaging aberrations in microlithography it is useful for optical symmetry properties of a projection exposure apparatus to be known accurately. In this regard it is observed, for instance, that when dipole illumination is used, overlay errors occur to an increased extent if the relation between the poles of the dipole, also designated as “pole balance”, is not balanced. In order to quantify such a pole balance, the pupil in the projection lens of the exposure apparatus is measured using a measurement mask having a pinhole stop, the aperture of which has a diameter of typically 80 to 100 μm. The intensity distribution generated in this case is imaged in a defocused manner onto a sensor in the vicinity of the wafer plane. With this measurement technique known to the person skilled in the art from the prior art, however, the pole balance can be measured only for the overall optical system of the projection exposure apparatus.
However, sensitivities with which unbalanced pole distributions affect overlay errors are dependent on whether the pole imbalance originates in the illumination system or in the projection optical unit. It is desirable, therefore, to measure the contributions to the pole distribution separately for different optical subregions of the projection exposure apparatus, such as, fir instance, separately for the illumination system and the projection lens.